1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices and particularly to a technique of forming grooves (streets) separating individual semiconductor devices in a semiconductor film formed over a metal support.
2. Description of the Related Art
Optical semiconductor devices such as light emitting diodes have advanced to be of high efficiency and high output power due to the recent years' progress in technology. However, with the output power having become higher, the amount of heat produced in the optical semiconductor device has increased. A decrease in the reliability of the optical semiconductor device due to, for example, a decrease in efficiency and degradation of the semiconductor film is posing a problem. In order to solve the problem, the semiconductor device is configured such that a growth substrate (i.e., a substrate used for crystal growth) which is relatively low in heat conductivity, is removed from the semiconductor film and that instead, the semiconductor film is supported by metal, which is relatively high in heat conductivity (Japanese Translation of PCT International Application Publication No. 2007-536725). With this structure, the thermal conductivity of the optical semiconductor device is improved, and in addition an improvement in the light extraction efficiency can be expected because of the removal of the growth substrate. That is, light absorption occurring when light passes through the growth substrate and total light reflection occurring at the interface due to the refractive index difference between the semiconductor film and the growth substrate can be reduced.
As to optical semiconductor devices comprising the metal support, before cutting a wafer into individual semiconductor devices by a laser scribing method or the like, grooves hereinafter, referred to as “streets” along multiple division lines and arranged in a lattice are formed in the surface of the wafer to make the metal support exposed at the bottom of the streets, and then the metal support is scribed by a laser scribing method or the like to divide the wafer into chips. The reason why the semiconductor film and the metal support are individually divided is that the effective method to partially remove the semiconductor film is different from the effective method to partially remove metal forming the metal support. For example, in the case of a blue light emitting diode having a semiconductor film made of AlxInyGazN (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1), the streets are formed by partially removing the semiconductor film along device division lines by dry etching such as RIE or wet etching using an alkaline solution such as KOH. Then, in order to cut the metal support partially exposed at the bottom of the streets, a technique such as laser scribing, dicing, or scribing/breaking is used.
As described above, in the conventional manufacturing process of semiconductor devices having a metal support, the streets are formed by partially removing the semiconductor film along device division lines only by dry etching or wet etching. However, when the semiconductor film is etched only by dry etching until the metal support is exposed, plasma in the etching process will be impinges on the exposed metal support surface, and metal particles forming the metal support may scatter into the atmosphere and stick to the semiconductor layer surface. FIGS. 1A and 1B are SEM images showing the state of a semiconductor device where the streets are formed only by dry etching. It is ascertained from these SEM images that metal particles scattered from the metal support are sticking to the sides of the semiconductor film. Metal particles sticking to the side of the semiconductor film, especially at or near the active layer may cause a current at operation to flow through the sticking metal particles, resulting in a leak current or a short. This sticking metal is difficult to remove even by acid treatment or the like, and cannot be removed unless part of the semiconductor film is scraped off. It is practically impossible to control to finish dry etching just when the metal support is exposed so as to prevent the scattering of metal particles because of the flatness of the metal support. Even if the metal support is slightly exposed to plasma, metal sticks to the semiconductor film (FIG. 1B).
In order to avoid the above problem, the method which partially removes the semiconductor film by wet etching can be used. However, according to a study by the inventors of the present invention, it is made clear that performing wet etching on the semiconductor layer (n-type semiconductor layer) exposed by removing the growth substrate cannot completely remove the intended part of the semiconductor film, and that thus etching residue occurs. This is thought to be because, with wet etching using an alkaline solution such as KOH being anisotropic, there remains a crystal plane of the AlxInyGazN crystal which is relatively stable to the etchant and slow in etching rate. Further, after the growth substrate is removed by LLO (laser lift-off) and so on, the surface of the semiconductor has laser traces and dislocations produced at crystal growth with various crystal planes or faces being exposed and non-uniformly distributed. Thus, the surface is in a state where etching residue is more likely to occur. FIG. 2 is a SEM image showing a top view of a street formed in the semiconductor film only by wet etching. It is ascertained that etching residues occur along scanning lines of laser irradiation in the LLO process on a street. This is thought to be because the semiconductor film has partially deteriorated due to the laser irradiation in the LLO process, so that the etching rate of the deteriorated portions of the semiconductor film has decreased.
When laser scribing is performed to divide the metal support with etching residues occurring on a street, laser beam will be scattered by the residues of the semiconductor film on the street, and therefore the metal support cannot be cut. Although it is thought to be able to address the problem of etching residue by raising the temperature of the etchant or lengthening the process time in this method, the semiconductor film will be damaged and in addition the process time will become longer, which are not preferable. Further, a resist mask needs to be formed on the semiconductor film to protect areas not intended to be etched when forming the streets in the semiconductor film. However, the resist will be degraded, and in addition side etching under the resist may occur when wet etching is performed under conditions of relatively high reactivity as mentioned above.